# How to calculate the bending of seamless black steel pipe?

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Seamless black steel pipe is a steel pipe made by perforating the whole round steel without welds on the surface. Bending refers to the degree of bending of the steel pipe in the length direction, and the degree of bending is expressed in numbers. The curvature specified in the standard is generally divided into the following two types:

1) Local curvature: Measure the curved part of the steel pipe with a one-meter-long ruler to measure its chord height. The unit is mm, which is the local curvature value. The unit is mm/m, and the expression method is 2.5mm/m. Suitable for measuring the curvature of pipe ends.

2) The total bending degree of the whole length: Tighten both ends of the pipe with a thin rope, measure the chord height mm of the bending part of the steel pipe, and then convert it into a length percentage in meters, that is, the full length bending of the steel pipe in the length direction.

For example: the length of the steel pipe is 10m, and the measured maximum chord height is 30mm, then the bending degree of the whole length of the pipe should be:
0.03/10m*100%=0.3%

Uneven wall thickness of seamless black steel pipe: the wall thickness of the steel pipe cannot be the same everywhere, and there are uneven wall thicknesses in the cross-section and longitudinal pipe body. In order to control this unevenness, some steel pipe specifications stipulate the allowable index of uneven wall thickness, which generally stipulates that it does not exceed 80% of the allowable deviation of wall thickness (implemented after negotiation between the supplier and the buyer).

Out of tolerance: The specification of the seamless pipe exceeds the allowable deviation of the national standard. The deviation may be “positive” or “negative”, and it is rare for the same batch of steel pipes to have inconsistent “positive and negative” deviations. There is a phenomenon of unequal outer diameter on the seamless steel pipe section, that is, the outer diameter and the small outer diameter are not necessarily perpendicular, and the difference between the outer diameter and the small outer diameter is called ellipticity (or out of circle). In order to control the ovality, some seamless steel pipes stipulate the allowable index of ovality, which is generally stipulated as not exceeding 80% of the outer diameter tolerance (implemented after negotiation between the supplier and the buyer).